1. Field of the Invention
The present invention relates to a heater having a resistance-adjusting function and employing a honeycomb structure.
Honeycomb heaters of the above-described type can be employed as heaters for domestic use, such as hot-air heater, or as industrial heaters, such as preheaters used for control of automobile exhaust emissions.
2. Description of the Prior Art
These days, besides porous ceramic honeycomb structures conventionally employed, metal honeycomb structures are drawing attention as catalysts or catalyst carriers for removing nitrogen oxide, carbon monoxide and hydrocarbons present in the exhaust gas of internal combustion engines, such as from automobiles.
As restriction of exhaust emission has been intensified, there has been a demand for development of heaters for use in reducing automobile exhaust emission when the temperature of the catalyst is not sufficiently high, i.e., at the beginning of running of a vehicle.
Honeycomb structures have been proposed in, for example, U.K. Patent 1492929 and Japanese Utility Model Laid-Open No. 67609/1988.
U.K. Patent 1492929 discloses the use of foil-type metal honeycomb structure in a catalyst for use in automobile exhaust emission control. This honeycomb structure comprises a metal substrate produced by winding a flat plate with, a mechanically deformed, corrugated plate. This metal substrate has an aluminum oxide film formed on the surface thereof by the oxidation process. The catalyst for use in automobile exhaust emission control is manufactured by placing a large surface area oxide, such as alumina, on the aluminum oxide film of the metal substrate and by impregnating a noble metal on the large surface area oxide.
Japanese Utility Model Laid-Open No. 67609/1988 discloses an electrically conductive metal monolith catalyst comprising a metal support and alumina coated thereon, for use as a preheater.
In the foil-type metal honeycomb structure disclosed in U.K. Patent 1492929, however, the metal substrate with a coating formed thereon cannot be closely adhered to a catalyst layer because of its low porosity, and a ceramic catalyst readily peels off the metal substrate due to a difference in the thermal expansion between the ceramic catalyst and the metal substrate. Furthermore, telescope phenomenon readily occurs during the run cycle in which a metal-to-metal join breaks and the metal substrate is deformed in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil-type metal honeycomb, yield of the rolling process is low, inviting high production cost. In the preheater proposed in Japanese utility Model Laid-Open No. 67609/1988, a catalyst readily peels off due to a difference in thermal expansion between alumina and the metal support. Furthermore, a metal-to-metal join of the metal substrate breaks during the operation, generating an electrically insulating portion and, hence, non-uniform flow of current and non-uniform heating.
The preheater disclosed in Japanese utility Model Laid-Open No. 67609/1988 is constructed so that a current is supplied between the inner periphery and the outer periphery of the foil-type metal honeycomb structure to generate heat. However, the preheater is not arranged so that it has an adjusted resistance (that is, only the material, dimension and rib thickness of the honeycomb structure are fixed but a desired resistance cannot be adjusted), and therefore, exhibits insufficient raising-temperature characteristic. Furthermore, since the electrodes are provided on the inner peripheral portion of the preheater, the central portion thereof does not act as a catalyst and pressure loss may be generated. Furthermore, the electrodes readily break due to the flow of gas.